Effect of Cropping Systems on Activities of Phosphorus- and Sulfur-Transforming Enzymes in Soils.

As key indicators of soil functional capacity, activities of enzymes are affected by soil management practices and cropping systems. This study was carried out to elucidate the effect of long-term (>30 yrs) cropping systems on activities of acid phosphatase, alkaline phosphatase, phosphodiesterase, inorganic pyrophosphatase and arylsulfatase in the soils. The evaluated cropping systems included monocrop soybean (MS), double-crop wheat (for grain)-full season soybean (DC), double crop wheat (for grazing)-early season soybean (GS), and modified double-crop, a two-year rotation including MD1 (early season soybean-winter wheat-full season soybean and MD2 (full season soybean-winter wheat-early season soybean). Of the enzymes tested, alkaline phosphatase activities were the most sensitive, while the activities of inorganic pyrophosphatases were the least sensitive to different cropping systems. In fact, the activities of inorganic pyrophosphatase were not significantly affected by cropping systems following more than 30 yrs of treatment. Although the degree of impact on different enzyme activities varied considerably, activities of acid phosphatase, phosdiesterase and arylsulfatase were generally higher in MD2 than other cropping systems. Activities of alkaline phosphase were the highest in MS. GS resulted in the lowest average activities of the tested enzymes. The activities of all tested enzymes were significantly correlated with organic carbon (n=60), with r values ranging from 0.58^*** to 0.89^***, and with organic N from 0.57^*** to 0.84^***. The activities of acid phosphatase, alkaline phosphatase, and inorganic pyrophosphatase were significantly correlated with soil pH, with r values ranging from -0.38^*** to 0.52^***, but activities of phosphodiesterase and arylsulfatase had little relationship with soil pH. With the exception of alkaline phosphatase, activities of the tested enzymes decreased with increasing soil depth. Multivariate analysis revealed that activities of acid phosphatase, phosphodiesterse, arylsulfatase, total nitrogen and organic carbon contributed most to principle component one which explained 45.8% of the total variances. Redundancy analysis suggested that activities of arylsulfatase and phosphodiesterase were primarily affected by total nitrogen and organic carbon; while activities of alkaline phosphatase were positively affected by soil pH.